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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Lightning protection of local networks. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Computers One of the problems that developers of local computer networks have to face is ensuring the resistance of network equipment to various external influences. A special role is assigned to lightning protection devices. With the development of "home networks" this problem becomes really acute, since a large proportion of the equipment fails due to static electricity. The topic of lightning protection devices is traditionally one of the most discussed among radio amateurs and professionals and is shrouded in various myths and inaccuracies. The proposed article provides an answer to the question: is it possible to withstand the effects of lightning discharges that are too powerful for the equipment, and ways and methods for protecting active equipment are determined. Attempts to protect against lightning discharges were known long before our era. During archaeological excavations in Egypt, inscriptions were found on the walls of destroyed temples, from which it follows that the masts installed around the temples served to protect against "heavenly fire". The oscillatory nature of a lightning discharge was proven even before the experimental work of G. Hertz. It turned out to be important that, in addition to a significant electrostatic potential caused by the movement of water drops, dust particles and pieces of ice at high speed, a lightning discharge acts as a powerful radio transmitter that generates strong electromagnetic radiation. The spectral composition of this radiation lies in the range from several hertz to tens of kilohertz, the highest density of which is in the region of 5...8 kHz. For this reason, the transformer decoupling of devices from information lines made by twisted pair (LVVP) is often powerless. Huge power interference passes through the isolation transformer without destroying it, but damaging the electronics. Studies have shown that the duration of such pulses can range from 1 to 500 microseconds or more, and the voltage can range from hundreds of volts to tens of kilovolts. As a result of long-term studies by various laboratories of the world, averaged parameters of lightning discharge pulses were obtained. On power lines and telephony with a length measured in kilometers, voltage impulses up to 20 ... 25 kV and current up to 10 kA are possible. In shorter lines, hundreds of meters long, voltage pulses up to 6 kV and current up to 5 kA are induced, and in lines passing inside buildings - up to 6 kV and up to 500 A. According to the statistics published on the website , the percentage of "survival" of equipment that is connected to overhead lines made of unshielded twisted pair is only 2%. The figures obtained by the author while servicing the local network of one of the enterprises, as a whole, fully confirm what has been said. And the failure of equipment connected to coaxial cable lines is not uncommon even inside brick buildings. On such overhead lines, the equipment practically "does not live" without special protection measures. We note right away that there is no absolute protection against such impacts, but it is undoubtedly possible to minimize losses based on a reasonable compromise between the cost, complexity and effectiveness of protection devices. Of course, it's nice to use "classic" methods: switching to fiber optic cables, abandoning open lines, shielding the cable system, but sometimes all this is not available for medium and small networks due to the high cost and complexity of installation. So, consider the main causes of equipment failure during a thunderstorm. 1. The formation of static electricity on cables and equipment as a result of the influence of stationary charges accumulated in a thundercloud. Air lines are most susceptible to static charges. Moreover, a significant charge can also accumulate in dry weather in winter during snowfall and in summer during the so-called "sand blizzards". The main method of protection is to ensure the removal of static electricity by grounding the shield and (or) a conductive traverse and installing arresters at both ends of the cable. Here, the correct grounding and the reliability of the arresters, which are subject to high requirements for the removal of significant current, come to the fore. 2. Induction in the cable system of high voltage pulses that occur as a result of exposure to a powerful electromagnetic field generated by lightning discharges. If the LVVP used is not shielded, as a result of exposure to a powerful electromagnetic wave, a small voltage is induced at each twisting step, within a few millivolts. If the LVVP is made perfectly and the area of the circuits is the same, the total induced EMF is close to zero. In reality, the twisting pitch is far from being the same, so there is no complete mutual compensation of elementary EMFs, and the longer the cable, the higher the voltage between the conductors of one pair can be as a result of an electromagnetic pulse created by lightning. This voltage can reach several hundred volts. The main method of protection is shielding, installation of potential-equalizing protection devices at the ends of the cable, at which the maximum voltage between any two wires in the cable does not exceed 7 ... 10 V. A potential exceeding hundreds of volts relative to earth reduces the arrester. 3. Voltage surges in the mains. This is a fairly common reason for the failure of equipment "entirely". In a 220 V network, voltage surges of up to several thousand volts often occur. The reasons for this are the operation of fuses at the substation, lightning discharge, interference from other powerful energy consumers. Traditional methods of protection - increasing the reliability of standard power supplies, the use of uninterruptible power supplies and protection devices against overvoltage in the network. 4. Changing the potential of grounding devices. It occurs when a lightning discharge close to the earth's surface. The main reason for equipment failure is a large potential difference on the grounding buses of equipment installed at a considerable distance from each other. In this case, a very large equalizing current flows through the cable lines and I/O circuits, which destroys the electronic or electrical equipment. In this case, losses can be minimized by strictly observing the rules for installing grounding devices. One of the leading positions in sales is occupied by lightning protection devices (LG) for domestic use ProtectNet by ARS. However, with a very affordable price and external attractiveness, these HDL HAs are not without drawbacks. The metal oxide varistors used in them, although they have a high speed and a very low price, are not able to reliably protect equipment on unshielded overhead lines. The residual voltage on them can be several times higher than the maximum allowable for the protected equipment. This is explained by the non-ideal current-voltage characteristic of the varistors and the dependence of the voltage on the amplitude of the current pulse flowing through them. It should also be taken into account that the protective elements gradually change their parameters, degrade if a current close to the limit flows through them. In this case, the internal resistance of the varistors decreases and they, in the end, close the protected line. Almost after a couple of years of operation on overhead lines, the protective properties of devices are lost and losses increase, so it becomes impossible to use them in high-speed networks over long distances. In many domestically produced UGs, either neon lamps or "neon" lamps from fluorescent lamp starters are used as arresters. This is mainly due to the low cost of such protective elements. In the opinion of the author, such a solution is not very successful, since neon lamps have high breakdown resistance and low speed. Long-term testing of an unshielded HDTV 100-megabit network with a length of one hundred meters, stretched between buildings, showed that the device, the diagram of which is shown in Fig. 1. It is a multi-phase diode bridge based on VD1 VD16 diodes, the diagonal of which includes a protective diode VD17, which limits the voltage between any two line conductors at a level of about 8 V. The use of Transil limiting diodes is due to significant differences in the parameters of such devices from zener diodes. For example, the response time of the clamping diode is less than a few picoseconds, and the peak power dissipation (within 1 ms) is 1500 W.
A line is connected to the XS1 connector, and network equipment is connected to the XS2 connector. The cable connecting the UG to the network equipment must be of a minimum length. Each conductor of the information cable is connected to the ground through gas-filled arresters F1-F4, which ensure the removal of static electricity potential exceeding 90 V. Specialized arresters Epcos T83-A90X allow the passage of a pulsed current of 10 kA with a duration of 8/20 μs, characteristic of a lightning discharge. Dual arresters are used on the basis of economic considerations only, instead of them, you can use any that meet the above requirements. Instead of diodes 1N4007 (VD1-VD16), you can use any similar rectifier diodes of imported and domestic production with a permissible reverse voltage of at least 1000 V, capable of operating at frequencies above 10 kHz. The UG is assembled on a printed circuit board made of double-sided foil fiberglass with a thickness of 1,5 mm. A drawing of the printed circuit board of the device is shown in fig. 2.
The foil on the board on the side of the elements acts as a screen; it is removed only near the leads of the parts, countersinking the holes. The middle terminal of the arresters is soldered directly to the foil from the side of the parts. The ground conductor is inserted into a hole with a diameter of 2 mm and soldered to both sides of the board. To reduce crosstalk, jumpers 1 and 2,3, 6 and 4, 5 and 7, 8 and 3 can be twisted in pairs with two or three turns. The appearance of the assembled UG board is shown in fig. XNUMX.
The device is mounted in a standard double socket RG45B (Fig. 4).
Since the numbering of the XS1 and XS2 connector pins is reversed relative to each other in this socket, jumpers had to be used on the printed circuit board. In the case of another mounting option, the UG jumpers can be excluded. The regular knife connectors are removed from the socket board, and curved pins are soldered instead (Fig. 5), on which the UG board is mounted (Fig. 6).
If there is no need to protect all eight conductors of the cable, the UG can be assembled according to the simplified scheme shown in fig. 7. Unused conductors are connected together and connected to ground through the arrester F2 (Epcos N81-A90X).
To protect power sources from short bursts of voltage in the 220 V network, a device is used, the circuit of which is shown in fig. 8. It is included in the break of the mains wire as close as possible to the power supply, for example, it is built into a mains outlet.
If the length of the low-voltage (9 ... 12 V) power supply circuit of the equipment is several meters or more, for example, power is supplied through free pairs or unshielded wires, then it is necessary to install a UG, which is assembled according to the scheme of Fig. 8, characterized in that instead of two, only one 1.5KE18 clamping diode is used, connected by the cathode to the power plus. The device is connected as close as possible to the active equipment in a break in the low-voltage DC power circuit. All types of UG require a mandatory connection to ground or protective neutral, we will assume that this, in our case, is one and the same. In its absence, all measures for lightning protection are practically reduced to zero. Let us dwell on the main points regarding the connection of the UG to ground. According to the Electrical Installation Rules (PUE), the electrical network in residential buildings consists of a phase (L), a working zero (N) and a protective zero (PE), connected to the switchboard housing on the landing and the middle contact of the outlet in the apartment. If your house was built after 1998, then with a high degree of probability it can be assumed that a protective zero has been connected to the outlets. You can check for its presence by connecting an incandescent lamp for a voltage of 220 V relative to the phase, first to the neutral wire, then to the middle contact of the outlet. In both cases, the lamp should burn brightly and evenly, if the residual current device (RCD) in the shield is triggered when the lamp is connected to the middle contact, this will only confirm the presence of a protective zero If the protective zero is not brought into the room, you will have to carry it out yourself. This will require a wire with a cross section of at least 1,5 mm2, the larger the better. One end of the wire is fixed under any free bolt of the busbar connected to the switchboard housing, the other end is connected to the grounding contact of the socket or UG. It is not permissible to use a heating battery or water pipes as a protective zero. One of the reasons is the high resistance of such "grounding". In addition, in some cases, the potential on the battery may be different from zero, for example, if a neighbor uses pipes as a working zero due to a break in the neutral conductor in the wiring, which is strictly prohibited. And although in the basement of a building there should theoretically be a potential equalization system, in practice there is anything. If everything is more or less clear in city apartments, then it is not easy for owners, for example, of rural houses to decide on the right choice of protective grounding. Usually, 220 V voltage is supplied to rural houses by overhead power lines, and it is dangerous to use working zero as a protective one. In the event of an emergency (breakage of the neutral wire on the power line, a tree falling on the power line, etc.), a non-zero potential may appear on the neutral wire, up to phase voltage. In this case, natural ground electrodes can be used as a protective earthing device. Paragraph 1.7.70 of the PUE states in this regard: “It is recommended to use as natural grounding conductors: water and other metal pipelines laid in the ground, with the exception of pipelines of flammable liquids, flammable and explosive gases and mixtures, sewerage and central heating; casing pipes of wells; metal and reinforced concrete structures of buildings and structures in contact with the ground, metal shunts of hydraulic structures, conduits, gates, etc., lead sheaths of cables laid in the ground Aluminum sheaths of cables are not allowed to be used as natural grounding conductors. the only grounding conductors, then in the calculation of grounding devices they should be taken into account when the number of cables is at least two; grounding conductors of high-voltage lines (VL) supports connected to the grounding device of the electrical installation using an overhead lightning protection cable. if the cable is not isolated from the overhead line supports; neutral wires of overhead lines up to 1 kV with repeated grounding with the number of overhead lines at least two; rail tracks of the main non-electrified railways and access roads in the presence of a deliberate arrangement of jumpers between the rails. I would also like to note that according to the PUE "it is not allowed to combine zero working and zero protective conductors of various group lines ...", i.e. it is necessary to ground (zero) conductive traverses, cable suspension cables and unused conductors in the cable only from one end . The fact is that with a close lightning discharge into the ground, the potential of grounding devices changes significantly, as mentioned above. In addition, the potential difference between distant ground points can be very large and with a "hard" ground at both ends, a significant equalizing current can flow through the cables and equipment. UG supply and information lines, similar to those described, can be used not only to protect HDPE, but also telephone, fire and burglar alarm lines, video surveillance systems and other information and supply lines of active equipment remote at a distance of more than several tens of meters, especially those operated outdoors. air. Author: D.Malorod, Kovrov, Vladimir region
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